Satellite navigation systems allow electronic receivers to determine navigational information such as position (latitude, longitude, and altitude), velocity and time, also known as PVI information. One example of such a system is the United States Naystar Global Positioning System (GPS), which may include up to thirty-two or more functional navigation satellites. Other examples of satellite navigation systems include the Russian GLONASS system and the European Galileo system. Satellite navigation receivers, such as GPS receivers typically use GPS data from three or more orbiting satellites to determine navigation information. Only a portion of the satellites within a navigation system may be visible to a particular navigation receiver at a given time.
GPS satellites typically transmit GPS signals on two bands: the L1 band with a carrier frequency of 1575.42 MHz and the L2 band with a carrier frequency of 1227.60 MHz. Traditionally, only authorized users have been able to use data transmitted on the L2 band. In the future, civilian GPS signals may be transmitted on the L2 band and the L5 band (1176.45 MHz). Typically, low cost GPS receivers receive only on one of these bands. Some civilian GPS receivers may use clock data from the L2 band to refine GPS data carried in the L1 band. The following descriptions use the L1 band to describe exemplary embodiments; however, other embodiments may be implemented using one or more GPS bands or other global positioning signals.
GPS satellites transmit data using a form of spread spectrum coding known as code division multiple access (CDMA). Each satellite may be assigned a coarse acquisition (CA) code that resembles pseudo random noise and is typically unique to that satellite. Each satellite encodes data using the satellite's CA code and transmits encoded data on the L1 carrier frequency (i.e., data is spread using the CA code). Thus, all satellites are simultaneously transmitting data on a shared carrier frequency. In some embodiments, a ground-based pseudo-GPS satellite (i.e., a pseudo-lite) may transmit GPS data by using a CA code not used by any satellites or of a satellite that may be out of view of the GPS receiver. Once a GPS signal with a particular CA code is received and identified, the GPS receiver is said to have “acquired” the GPS satellite associated with that CA code. A GPS receiver may also “track” a GPS satellite by continuing to receive a GPS signal from a previously acquired GPS satellite.
The conventional approach to using GPS satellites for user positioning requires the receiver to download the navigation message from 4 or more visible satellites in order to determine an adequate PVI solution. The navigation message from each satellite contains the broadcast ephemeris, the ionospheric models, and UIC-GPS clock correction that are necessary for the user to compute the position of the satellites in the earth-centered earth-fixed (ECEF) coordinate system for a specified time.
The navigation message is formatted into flames. Each flame consists of 5 sub-frames that are 6 seconds each in duration. The broadcast ephemeris (including UIC-GPS clock correction) for each satellite is provided in sub-frames 1, 2 and 3, and the almanac and ionospheric models are provided in sub-frames 4 and 5. Consequently, the minimum time required to download the ephemeris data for a satellite is 18 to 36 seconds. The broadcast ephemeris is valid for a period of 4 hours starting from the time the satellite starts to broadcast the data.
Typically, after a GPS receiver acquires four or more GPS satellites, the GPS receiver may determine a PVI solution. If the GPS receiver can acquire more than four GPS satellites, the PVT solution may be made more accurate. Some GPS receivers may determine a PVT solution with less than four GPS satellites. Such a solution, however, may not be as accurate as a solution determined with four or more GPS satellites.
However, as one of skill in the art will appreciate, if the GPS receiver is turned on for the first time or after a few days, also known as a cold start, or after several hours from the last use, also known as a warm start, the receiver will not have the latest broadcast ephemeris from the navigation satellites. In this case the receiver has to wait until at least 4 satellites have been acquired and their broadcast ephemerides have been extracted before estimating the user position. This time period is generally known as the time to first fix (TTFF). As noted above, receiving ephemerides from the satellites necessarily introduces a latency of at least 30 seconds and up to several minutes before navigational determinations can be made. Moreover, under weak signal conditions, the signal to noise ratio of the transmission from one or more satellites may fall below the receiver's threshold to reliably decode the navigation message. Thus, the receiver may not be able to generate a position even after a period of minutes.
To improve TTFF and provide receiver operation when satellite reception is degraded, attempts have been made to supplement the ephemeris data available to the remote navigation device. In general, these methods use an external source of ephemeris data, independent of the satellite transmission, which is then delivered to the receiver.
For example, a first method is known as Assisted GPS or AGPS In AGPS, the navigation device maintains a wireless communication with the external source of the ephemeris data. Naturally, this method requires that the navigation device maintain a wireless communication link, such as over a cellular telephone network, to a server in order to receive ephemeris assistance on demand. Alternatively, a second method is known as extended ephemeris, wherein a server generates predicted pseudo-ephemeris data for one or more satellites having a validity of several days and formats the prediction into a data file. This file is periodically transmitted to the navigation receiver over a wireless medium, such as the cellular telephone network, or via a wired connection, such as through a personal computer linked to the Internet.
Although these methods can be used to facilitate navigational determinations when some portion of broadcast ephemeris data is unavailable to the receiver, they require an additional source of ephemeris data and either a continuous or intermittent wireless link or the periodic connection of the receiver to a network to deliver the ephemeris data. Accordingly, there is a need for systems and methods that reduce the TTFF without requiring an external source of ephemeris data. Similarly, there is a need for systems and methods that can provide position information when signal reception from the navigational satellites has been compromised.